The present invention relates to apparatus for constructing a security barrier for preventing or controlling vehicle access along a roadway or path. The barrier sometimes includes an elongated vehicle capture net which is connected on each end to an elongated flexible restraining member such as a flexible strap or cable or reinforced rope which is pulled out from a brake controlled drum or spool, for example, as disclosed in U.S. Pat. Nos. 7,125,197 and 7,736,084. The apparatus disclosed in these patents uses straps which extend from spools, and rotation of the spools is controlled by a dry friction brake system having brake disks or caliper disks which brake the shaft supporting the spools on which the flexible straps are wound. As disclosed in U.S. Pat. No. 7,125,197, unwinding of the flexible restraining member or straps from the spools advances the spools on threaded portions of the shaft to compress the adjacent dry friction braking surfaces against each other. The vehicle is restrained or stopped by deploying the vehicle capture net which is connected on each end by a pair of straps wound on corresponding spools. When the vehicle collides with the capture net, the straps unwind from the corresponding spools and provide a restraining force of increasing magnitude on the straps.
High power absorption rates are required from each brake due to the short operating time of the brake. The dry friction brakes used under these conditions are subject to accelerated wear due to inherent thermal ablation of the friction surfaces. As the friction surfaces wear, the brake capacity to absorb the vehicle kinetic energy is diminished, and this often requires replacement of the friction surfaces after a single operation or use of the brake.